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July 10, 1928.

L. M. HULL VACUUM TUBE cmcums Filed Jan. 17, 1927 2 Sheets-Sheet 1 Suva tot:

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July 10, 1928.

L. M. HULL VACUUM TUBE CIRCUITS Filed Jan. 17, 1927 2 Sheets-Sheet 2 Patented July 10, i928.

UNITED s'rniss PATENT OFFICE.

LEWIS fir. HULL, or noou'ron, NEW JERSEY, assreuon TO RADIO FREQUENCY nano- RATQEIES, mconrona'rsn, or BOSTON, MASSACHUSETTS, A conronn'rron or mew JERSEY;

VACUUM-TUBE CIRCUITS.

Application filed Januar 17, um. Serial No. 161,66L

electrode type.

Such a vacuum tube is ordinarily operated in connection with an input circuit connected between 'the filament and the control grid, and an output circuit connected between the plate and the filament. It is well known that coupling between the output, and input circuits' may cause undesirable retroactive or feed-back currents, and even self-oscillation. This coupling-may be through the inherent tube capacities, and leads, and so forth, but capacity? between the control grid and the plate of the vacuum tube is largely responsible for the undesirable. currents due to coupling-between the two cir euits, even in tubes of the 'foiir-electrodc type,

Various methods of compensating the effects of" this giid-pl'ate rapacity have been proposed;.for example-feeding back a compensating currentorito the grid' throu'gh a compensatin capacity in-serieswith a coil closely coup ed to acoil in the input or electrode vacuum tube in which a positivelybiased emission-shield grid employed. The impedance of the. output circuit should, of course, be approximately equal to the in- 'ternal impedance ofthe tube, and inthe case of a four-electrode tube this may be of the order of, say, 250,000ohms. If an output circuit of this type includes a closely coupled compensating coil the resistance and eddy current losses areoften so high as to offset the advantages which might otherwise arise from the use of a high-impedance-circuit.

My invention applies to vacuum tubeam .plifiers' of both the three-electrode and the four-electrode type, but-is. of particular advantage in connection with amplifiers-eniploym'g a high impedanceinput or outputcircuit, as in some applications of the fourelectrode tube. It obviates the necessity for employing closely coupled coils in the input. or output circuit, and consequently prevents losses and reduction of amplification due'..to increased resistance and to eddy currents.

These and other undesirable effects in all hitherto known amplifier systems, which are balanced .to suppress interaction between in.- put and output circuits, are, so. far as I am aware, largely due to the fact thatthe fdisturb ing couplings inherent in such systems are essentially capacitive, consisting, as previous- I ly described,.of internal tube capacities aug+ mented by external stray capacities. I have found that it is possible to simplify the problem of suppressing interaction between.

the input and output circuits by'so changing the essential character of this inherent cou-'. phng between the circuits. t'hat'itis no longerpurclycapacitive within theioperating fre.-

quency, range. I accomplish this by the ad- -ditiori to a vacuum tube amplifier of an'inductive element. which is so related totlieinput and output circuits as-to cause'the retro activeor feed-back currents to flow through an inductance in series with a..caipacit-y, in-v stead of through a capacity alone. This I- may accomplish by including a self-induc-' -tance in the connection between the high-p0,-

tential side of the input circuit (i. c. the side which is aha high alternating current potential with respect to the filament) and the control grid of the vacuumtube, I can then impress a compensating potentialupon either-the high-potential terminal of the input circuitor upon the plate bymeans'of'a series circuit including a compensating condenser and a coil having relatively loose coupliug with a coil in either the input or the output circuit, as may be appropriate. I may thus obtain a substantial or complete compensation of theumd'csirable or disturblu potentials caused by the How of currents through grid-plate capacity of the tube. I

have obtained satisfactory resultswith a S39 pling of the order of about 50% to 60%. Even in the case of a high-impedance input or output circuit this degree of coupling does not necessarily cause an undesirably high resistance, and does not injuriously reduce the amplification.

Figure 1 illustrates one embodiment of my invention. Figure 2- illustrates ahother embodin'ient thereof. Figure 3 shows an embodiment of my invention including a four element oriour-electrode vacuum tube. Figure 4 shows another embodiment of my invention including such a tube. Figure 5 shows a. cascaded arrangement employing circuits similar to those shown in Figure 3.

In Fig. 1 I have shown my invention applied to a vacuum tube of the three-electrode type. A tuned input circuit, L,-C

is connected between the two input terminals 1 and 2. Terminal 2 is connected to the filament F, a C battery being used if desired. The high-potential input terminal, 1, is connected to the grid of the tube through the self-inductance L0. Cm represents capacity between grid and plate of the tube. plate inductance, L is connected between plate and filament of the vacuum tul eandint y.be cou led to a tuned circuit 30.

or .tuned transformr secondary, L,-C The B battery and,by-pass condenser may be arranged as usual.- Compensatin coil L, is coupled to coil L in the input circuit. There is a series connection from L, through compensating coil L and compensating condenser Co to the plate. Coils L, and L are coupled to ether with relatively loose coupling, whic may suitably be of the order of about 50% to 60%, and in such a sense as to impress upon the grid a compensating potential of the proper phase to compensate the disturbing potential due to coupling through capacity Cm and inductance L0 in series therewith. C0 or other circuit element may be adjusted to give the proper am plitude to this compensating potential.

While my invention is not dependent upon theory, I believe that the mathematical conditions for complete compensation, in the arrangement of Fig. 1, may be expressed as follows:

where the symbols represent the numerical magnitudes of the iuductances and capacities of the several circuit elements.

In Figure 2 I have also shown my invention applied to a three-electrode tube, hut. in this case the compensation is brought about by means of a series circuitfrom the high-potential input terminal, 1, through compensating condenser C0 and compensating coil L to the plate coil L L and L may be coupled relatively loosely, as before,

In Fig. 3 I have shown a four-electrode vacuum tube amplifier involving the arrangen'ient of Fig. 1. In addition to the compensation effected by the series circuit Off-L this arrangement has the advantage that by connecting the positively-biased emission-shield grid, G to the filament through coil L an alternating potential is impressed upon grid Gr in-such phase as to cause it to exert an auxiliary control effect, and thus modify the amplification of the vacuum tube in a predetermined manner, the extent of this modification depending upon the control constants of the tube and the relative. values chosen for the inductances L,, L, and L,. I

In Fig. 4 I have shown a four-electrode vacuum tube amplifier involving the arrangement of'Fig. 2. By this arrangement also the compensating coil L is caused to exert an auxiliary control eflect, as in the arrangement of Fig. 3.

not

In Fig. 5 I have shown two four-electrode vacuum tube amplifier stages, I and I], each involving the arrangementof Fig. 3, Working into a detector stage III.

It is obvious that a vacuum tube amplifier embodying my invention may vary considerably from the specific arrangements herein set forth. The invention extends to all ar-- rangements in which a self-inductance is connected between the primary control element and the highrpotential input terminal, in order to assist in the nrpensathm at vacuum tube amplifier. or to improve the eration thereof.

I claim:

1. An electrical. amplifier including a. vac uum tube, an input circuit, an output circuit, elements producing capacitive coupling b tween said circuits, a circuit for com nsat series a compensating con coupled to one of said circuits and a compensatin condenser connected to the other of id circuitfl a'litl an inductance connected to a. control element and control means, an input circuit connected between said control means and said cathode, an output circuit connected between said anode and said cathode, a compensating circuit connected between said input and output circuits and including in series a compensating coil-coupled to one of said circuits and a compensating condenser connected to the other of said circuits and an inductance connected between the high potential termiml of said input circuit and said control means.

3. An electrical amplifier circuit comprising a four-electrode vacuum tube including anode, cathode',--.control grid and a secon grid, an inputcircuit connected between said control grid and said cathode, an output circuit connected between said anode and said cathode, a compensating circuit including an inductance and a capacity connected in series between said inputv and output circuits, said inductance being coupled to an inductance in one of saidcircuits, aconnection from said second grid to said cathode through said compensating circuit coil to produce an auxiliary control efiect, and an inductance connected between the high-potential terminal of said input circuit and said control grid.

4..An electrical amplifier circuit comprising a vacuum tubeincluding a cathode and at least two 'higii potential electrodes of which one constitutes an anode and another constitutes a control element; a circuit connected between said cathode and one of said high-potential electrodes; a second circuit havin one terminal connected to said catho e and another terminal connected through an inductance to another high-potential electrode to provide a composite inductive and capacitive coupling between said input and output circuits; and a compensating circuit forcompensating said composite couplinglcomprising a coil coupled to a coil in one oi said circuits between said cathode and a high-potential electrode and connected through a condenser to another high-potential electrode.

5. An electrical amplifier circuit comprising a vacuum tube including at least an anode, a cathode, and a control electrode; an input circuit associated with said cathode and control electrode; an output circuit associated with said cathode and anode; and means for compensating capacitive coupling between said output and input circuits comprising an inductance connected in series be tween one terminal of said input circuit and said control electrode and a capacity connected between a terminal of said inductance and a coil coupled to a coil of said output circuit.

6. An electrical amplifier circuit comprising a vacuum tube including an anode, a cathode, a control grid and a second grid; an input circuit connected between said control grid and said cathode and including an in-' ductance; an output circuit connected between said anode and said cathode and including an inductance; a compensating circuit connected between said input and output circuits and including in series a compensating coil coupled to one of said inductances in one of said circuits and compensating capac ity connected to the other of said circuits; an inductance connected between the high potential terminal of said input circuit and said control grid; and means for imparting a positive bias to said second grid.

7. An electrical amplifier circuit comprising a vacuum tube including at least an anode, a cathode and a control element; an input circuit having one terminal connected to said cathode and including an inductance; an output circuit associated with said tube and including an inductance; a third induc-- tance connected between the high potential terminal of said input circuit and said control element; and a compensating circuit connected between said input and output circuits and comprising capacity connected to one of said circuitsand a fourth inductance coupled to said inductance in the other of said circuits, Y

In testimony whereof, I aflix my signature.

LEWIS M. HULL. 

